Process for producing biodiesel, lubricants, and fuel and lubricant additives in a critical fluid medium

ABSTRACT

A process for producing alkyl esters useful in biofuels and lubricants by transesterifying glyceride- or esterifying free fatty acid-containing substances in a single critical phase medium is disclosed. The critical phase medium provides increased reaction rates, decreases the loss of catalyst or catalyst activity and improves the overall yield of desired product. The process involves the steps of dissolving an input glyceride- or free fatty acid-containing substance with an alcohol or water into a critical fluid medium; reacting the glyceride- or free fatty acid-containing substance with the alcohol or water input over either a solid or liquid acidic or basic catalyst and sequentially separating the products from each other and from the critical fluid medium, which critical fluid medium can then be recycled back in the process. The process significantly reduces the cost of producing additives or alternatives to automotive fuels and lubricants utilizing inexpensive glyceride- or free fatty acid-containing substances, such as animal fats, vegetable oils, rendered fats, and restaurant grease.

This application claims priority from PCT application Ser. NoPCT/US99/16669 filed Jul. 22, 1999, published internationally under PCTArticle 21(2) in English, which claims priority from United Statesprovisional application Ser. No 60/094,076 filed Jul. 24, 1998.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States has rights in this invention pursuant to Contract No.DE-AC07-94D13223 between the U.S. Department of Energy and LockheedMartin Idaho Technologies Company.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing biofuels asalternatives or additives to currently used petroleum-based automotiveor other vehicular fuels and lubricants by reacting fats and oils suchas triglycerides and free fatty acids in a single critical phase toprovide increased reaction rates and decrease the loss of catalyst orcatalyst activity. The invention includes the transesterification oftriglyceride-containing substances and esterification of free fattyacid-containing substances with alcohol to produce alkyl esters oftriglycerides, a desirable additive or alternative for petroleum dieselfuel or lubricants.

2. Description of the Prior Art

Significant quantities of esters such as triglycerides and free fattyacids are available from inexpensive feedstocks such as, animal fats,vegetable oils, rendered fats, restaurant grease and waste industrialfrying oils. The triglyceride esters can be reacted, or transesterified,with alcohol to produce glycerol and the alkyl esters, and the freefatty acid can be reacted, or esterified, with alcohol or water toproduce the alkyl ester. These alkyl esters create desirable additivesor alternatives to petroleum diesel fuel as well as other high value endproducts such as detergent surfactants, herbicides, pesticide diluents,sticking agents, or lubricating additives for hydraulic and transmissionfluids to name a few. Consequently, numerous patents exist dealing withprocesses surrounding transesterification of triglycerides andesterification of free fatty acids with alcohols such as methanol,ethanol or butanol to create the corresponding alkyl esters. U.S. Pat.Nos. 5,713,965 and 5,525,126, incorporated herein by reference, areexamples of such processes.

The transesterification or esterification reaction is normally carriedout in an excess of the stoichiometric quantity of alcohol and acatalyst, usually a base such as potassium hydroxide although, thereaction can also proceed with an acid catalyst as well. In addition tocreation of the alkyl esters, the transesterification reaction alsoproduces glycerol.

Traditionally, triglyceride transesterification requires a multiple stepprocess with one or more batch reactors. Initially, the triglyceridesand alcohol form two immiscible liquid phases. As the reaction proceeds,two separate liquid phases form. One contains the newly formed alkylesters of the triglyceride and the other the glycerol with the excessalcohol, catalyst and feed oil being dispersed into both phases. Thereaction time for each step typically is measured in hours and oncecompleted the liquid products must be allowed sufficient time toseparate phases before additional processing and separation steps canoccur to produce the final products. Excess alcohol must be recycled andthe unused catalyst typically must be neutralized.

Even the most efficient of the traditional processes require multiplehours to process each batch of feed. Additionally, significant problemsarise in the separation steps. Significant quantities of glycerol leftin the alkyl esters diminishes the quality of the diesel fuel andlikewise contaminated glycerol also loses much of its value compared topure uncontaminated glycerol. Traditionally the separation proceduresnecessary to adequately clean the two product streams produces largequantities of waste water thereby creating additional cost and/orprocess complexity.

SUMMARY AND OBJECTS OF THE INVENTION

This invention provides a single-phase process for producing alkylesters useful as biofuels and lubricants by the reactions oftriglyceride esters and free fatty acids. This invention teaches aprocess where the reactants enter a reactor, whether batch orcontinuous, dissolved in a critical fluid. The critical fluid provides asingle-phase medium in which diffusion of the reactants into differentliquid phases is eliminated, and mass transfer limitations areessentially eliminated thereby increasing the overall reaction rate.

A critical fluid is a fluid whose temperature is within 20% of thecritical temperature of the fluid as measured in Kelvin and pressurewithin 0.5-15 times the critical pressure as modified by any co-solvent.

Additionally, the solubility of the reaction products such as alkylesters in the critical fluid can be controlled by controlling thereactor's temperature and pressure. Where a reaction product'ssolubility is low or nonexistent such as glycerol it drops out of thefluid as it is created, thereby driving the reaction equilibrium towardproduct production which significantly reduces the quantity of excessreactants such as alcohol needed to drive the reaction to completion.

The use of a critical fluid also allows for a wide range of catalysts,both liquid phase and reusable solid phase acid or base catalysts. Solidphase catalysts have significant additional advantages by limitingunwanted side reactions and producing higher conversion rates of thedesired products.

With the reaction completed, the critical fluid medium also facilitatesclean, efficient separations. The reaction products typically can besequentially and selectively removed from the critical fluid medium byadjusting the temperature and pressure of the critical fluid medium. Ina transesterification reaction of triglycerides, the glycerol is removedfirst leaving the alcohol esters in the critical fluid. With theglycerol removed, the temperature and pressure is again changed to dropout the alkyl esters. Once the products are removed the critical fluidand any excess reactants are returned to the beginning of the process.

It is the object of the present invention to provide a process forreacting triglyceride- and free fatty acid-containing substances capableof completing the reaction in significantly less time than presentconventional processes.

It is another object of the present invention to provide a process forreacting triglyceride- and free fatty acid-containing substances capableof using reusable catalyst thereby avoiding the need for processneutralization.

It is yet another object of the present invention to provide a processfor reacting triglyceride- and free fatty acid-containing substancescapable of using a solid phase catalyst.

It is yet another object of the present invention to provide a processfor reacting triglyceride- and free fatty acid-containing substanceswith higher yields of desired product over conventional processes.

It is yet another object of the present invention to provide a processfor reacting triglyceride- and free fatty acid-containing substancescapable of operating in a single phase, thereby eliminating immiscibleliquid phases and reducing mass transfer reaction limitations.

It is yet another object of the present invention to provide a processfor reacting triglyceride- and free fatty acid-containing substanceswith improved separation capabilities for separating purified reactionproducts without the need for washing steps.

It is yet another object of the present invention to provide a processfor reacting triglycerides and free fatty acids which can quantitativelyreact the triglycerides or free fatty acids with significantly lessexcess alcohol than present processes.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained, a more particular descriptionof the invention briefly described above will be rendered by referenceto specific embodiments thereof which are illustrated in the appendeddrawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be considered tobe limiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is a simplified flow diagram which illustrates an embodiment ofthe invention employing a continuous reactor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention describes a process for reacting organic compoundshaving the generic formula R₁COOR₂ with short-chain (C₁ to C₄) alcoholsin a single critical fluid phase medium over an acidic or basic catalystto produce alkyl esters suitable as a biodiesel fuel. One skilled in theart will recognize that organic compounds from the family of compoundsknown as acylglycerols, fats, oils, waxes, or free fatty acids have thegeneral formula R₁COOR₂. One skilled in the art will also recognize thatacylglycerols can be mono, di, or tri substituted in any manner. Theinvention includes the transesterification and esterification of R₁COOR₂molecules with short-chain alcohols such as methanol, ethanol, propanol,or butanol to produce alcohol esters. The invention also encompasses theacid and base catalyzed hydrolysis of R₁COOR₂ compounds in a singlecritical fluid medium where R₁ is a chain of aliphatic hydrocarbons (C₄to C₃₆), and where R₂ can either be glycerol or can be another aliphatichydrocarbon chain (C₄ to C₃₆) linked to R₁ via the carboxylic ester(BCOO—) linkage. The reaction is generally described as follows:R₁COOR₂+R₃OH---->R₁COOR₃+R₂OH

Catalyst

In the case of mono-, di-, and triglycerides, fats and oils, R₁ is thealiphatic hydrocarbon (C₄ to C₂₄) chain, R₂ is glycerol and R₃ is ahydrocarbon group, and preferably short chain alkyl group, morepreferably methyl, ethyl, propyl, or butyl group attached to thealcohol. In the case of free fatty acids, R₁ is the aliphatichydrocarbon chain wherein the terminal carboxylic acid group could beR₁COO⁻, or R₁COOH, or R₁COO⁻ M⁺ where M⁺ is a metal (thus, the salt of afatty acid); and, R₃ is the short-chain hydrocarbon attached to thealcohol. In the case of a wax, R₁ is an aliphatic hydrocarbon chainlinked to R₂ via the carboxylic ester linkage, and R₃ is the short-chainhydrocarbon attached to the alcohol.

FIG. 1 provides a basic flow diagram for the glyceride reaction processemploying a continuous reactor. Throughout the description of theprocess diagram, the various process vessels will be numbered between 1and 99, with the various process streams being numbered beginning with100. While the following discussion will describe a continuous reactorprocess for a transesterification reaction of glycerides with an alcoholROH, one skilled in the art will recognize the process principles applyequally well in other process settings such as ones using batch reactorsand separation processes as well as reactions producing alternateproducts to the alcohol esters as well as processes beginning with fattyacid feeds.

A glyceride containing feed 100 is mixed with an input alcohol stream102. The choice of alcohol will be a function of the desired reactionproduct, and typically such alcohols as methanol, ethanol, propanol andbutanol are chosen for practical reasons, however, one skilled in theart will readily recognize the flexibility of choices and nonlimitingaspect of the above list. This input alcohol stream 102 containsapproximately a stoichiometric quantity of alcohol necessary toquantitatively react the input glyceride feed 100. While the reactionwill require some excess alcohol, that needed excess is contained in acritical fluid recycle stream 104 which provides a transport medium thatsolvates the reactants to create the required process conditions. Theexact critical fluid employed for a given reaction will depend onspecifically chosen process parameters such as temperature, pressure,desired reaction products, solubility of the reaction products, quantityof excess alcohol needed to drive the reaction to completion, postreaction separation processes and chosen catalyst. Examples of possiblecritical fluid solvents are carbon dioxide, sulfur dioxide, methane,ethane, propane, or mixtures thereof, with or without critical fluidco-solvents such as methanol, ethanol, butanol or water. Naturally, tothe extent quantities of the critical fluid are lost during the processthey can be replenished with a critical fluid make-up stream 106.

The mixing of the input feed 100 (substance containing free fatty acidsand/or glycerides), the input alcohol 102, the critical fluid recycle104 and critical fluid make-up 106 streams creates a reactant inputstream 108 which is fed into a continuous reactor 10. The temperatureand pressure of the reactant input stream will depend on its componentsand the desired process parameters. The important criteria for thecritical fluid is its ability to dissolve the reactants. Reactiontemperatures should be within 20% of the critical temperature of thefluid as measured in Kelvin, and pressures within 0.5-15 times criticalpressure as modified by any co-solvent. Reaction temperatures aretypically in the range from about 20 to 200 degrees C. with reactionpressures in the range from about 150 psig to 4000 psig.

The transesterification reaction generally proceeds in the presence of acatalyst, either acidic or basic. Liquid acids and bases, such as thecommon inorganic acids HCl, H₂SO₄ and HNO₃ and inorganic bases NaOH andKOH typically provide the needed catalytic activity. Additionally, theuse of a critical fluid medium allows for use of a solid phase catalystswith either acidic or basic surfaces such as microporous crystallinesolids, such as zeolites, and non-crystalline inorganic oxides such asalumina, silica, silica-alumina, boria, oxides of phosphorus, titaniumdioxide, zirconium dioxide, chromia, zinc oxide, magnesia, calciumoxide, iron oxides, unmodified, or modified with chlorine, fluorine,sulfur or an acid or base, as well as mixtures of the above group or anexchange resin with either acidic or basic properties. Where solidcatalysts are used in the reactor 10, they may create a catalytic packedbed or float free inside the reactor.

As the reaction proceeds, glycerol and the alkyl esters of the glycerideare produced. Glycerol has low solubility in critical fluids such as CO₂and propane which will cause the glycerol product to drop out of thecritical fluid medium. This removal of the glycerol from the reactionphase of the critical fluid medium will enhance the reaction equilibriumand drive the reaction further to completion with limited excessalcohol. One skilled in the art will then recognize that the quantity ofexcess alcohol required to drive the reaction to completion will dependnot only on such factors as the desired reaction rate, but also theglycerol solubility in the chosen critical fluid.

The ability of the critical fluid medium to solvate the reactantseliminates the immiscible phases found in conventional processes. Thesingle phase reaction eliminates inter-phase mass transfer of theindividual reactants and catalyst, thereby greatly increasing thereaction's rate.

After completion of the reaction the reaction vessel may be theseparation vessel, a final product stream 110 exits the reactor 10 andenters a first product separator 12. In the first product separator 12the product stream's temperature and/or pressure are modified to allowthe least soluble product in the critical fluid to quantitatively dropout, the glycerol in this embodiment. Once the glycerol has dropped outof the critical fluid medium, a physical separation of the two phasescan be readily accomplished. A glycerol product stream 112 and aglycerol depleted product stream 114 exits the first separator 12. Theglycerol depleted product stream 114 consists of the critical fluid,excess alcohol, alcohol ester of the glycerides and any remainingcatalyst, if a liquid catalyst is used, and then enters a second productseparator 14. Again the temperature and pressure of the critical fluidare lowered to allow the desired product, the alkyl ester of theglyceride of this embodiment, to drop out of the critical fluid whileretaining the excess alcohol in the critical fluid. The physicalseparation of the two phases then creates a second product stream of thealcohol ester 116 and the critical fluid recycle 104 which will bereintroduced back into the front of the process after having itspressure and temperature restored to the original input reactionrequirements.

While this embodiment describes a process with two product streams, theglycerol and alkyl ester, it should also be apparent to one skilled inthe art that a reaction producing more than two products can producemultiple product streams by simply increasing the number of productseparators.

Although the present invention has been described with reference topreferred embodiments, those skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A process for producing alkyl esters useful in biofuels andlubricants, the process comprising: providing an organic compositioncomprising at least one component selected from the group consisting:providing a critical fluid medium including at least one fluid selectedfrom the group consisting of carbon dioxide, sulfur dioxide, methane,ethane, propane, and mixtures thereof; dissolving the organiccomposition and either a C₁-C₄ short chain alcohol or water into thecritical fluid medium; reacting the organic composition with the shortchain alcohol or water in the presence of a catalyst in a single phaseto produce a final product comprising an alkyl ester and glycerol,wherein the glycerol leaves the single phase as it is formed; separatingthe glycerol from the alkyl ester; and separating the alkyl ester fromthe critical fluid medium, wherein the particular critical fluid mediumis selected so that, when combined with the organic composition, thecritical fluid medium provides decreased loss of catalyst or catalyticactivity and elimination of mass transfer limitations by maintaining thevarious reactants in a single phase.
 2. The process of claim 1, whereindissolving the organic composition and either a C₁-C₄ short chainalcohol or water into the critical fluid medium comprises dissolving theorganic composition and the short chain alcohol selected from the groupconsisting of ethanol, methanol, propanol, butanol, isopropanol andisobutanol into the critical fluid medium.
 3. The process of claim 1,wherein reacting the organic composition with the short chain alcohol orwater in the presence of a catalyst comprises reacting the organiccomposition in the presence of a liquid phase catalyst.
 4. The processof claim 3, wherein reacting the organic composition in the presence ofa liquid phase catalyst comprises reacting the organic composition inthe presence of the liquid phase catalyst selected from the groupconsisting of HCl, H₂SO₄, HNO₃, NaOH, and KOH.
 5. The process of claim1, wherein reacting the organic composition with the short chain alcoholor water in the presence of a catalyst comprises reacting the organiccomposition in the presence of a solid phase catalyst.
 6. The process ofclaim 5, wherein reacting the organic composition in the presence of asolid phase catalyst comprises reacting the organic composition in thepresence of a microporous crystalline solid.
 7. The process of claim 5,wherein reacting the organic composition in the presence of a solidphase catalyst comprises reacting the organic composition in thepresence of an exchange resin with either acidic or basic properties. 8.The process of claim 5, wherein reacting the organic composition in thepresence of a solid phase catalyst comprises reacting the organiccomposition in the presence of an inorganic oxide selected from thegroup consisting of alumina, silica, silica-alumina, boria, oxides ofphosphorus, titanium dioxide, zirconium dioxide, chromia, zinc oxide,magnesia, ion exchange resins, silicate catalysts, and calcium oxideeither unmodified or modified with chlorine, fluorine, sulfur or an acidor base.
 9. The process of claim 1, further comprising recycling thecritical fluid medium for use in a later reaction.
 10. The process ofclaim 1, wherein providing a critical fluid medium comprises providingthe critical fluid medium optionally including a critical fluidco-solvent selected from the group consisting of methanol, ethanol,butanol, and water.
 11. A process for producing alkyl esters useful inbiofuels and lubricants, the process comprising: providing an organiccomposition comprising at least one component selected from the groupconsisting: providing a critical fluid medium including at least onefluid selected from the group consisting of carbon dioxide, sulfurdioxide, methane, ethane, propane, and mixtures thereof, and optionallyincluding at least one critical fluid co-solvent selected from the groupconsisting of methanol, ethanol, butanol, and water; dissolving theorganic composition and either a C₁-C₄ short chain alcohol or water intothe critical fluid medium; reacting the organic composition with theshort chain alcohol or water in the presence of a catalyst in a singlephase to produce a final product comprising an alkyl ester and glycerol,wherein the glycerol leaves the single phase as it is formed; separatingthe glycerol from the final product by modifying the temperature andpressure of the final product; and separating the alkyl ester productfrom the critical fluid medium by modifying the temperature and pressureof the critical fluid medium.
 12. A process for producing alkyl estersuseful in biofuels and lubricants, the process comprising: providing anorganic composition comprising at least one component selected from thegroup consisting: providing a critical fluid medium including at leastone fluid selected from the group consisting of carbon dioxide, sulfurdioxide, methane, ethane, propane, and mixtures thereof; dissolving theorganic composition and either a C₁-C₄ short chain alcohol or water intothe critical fluid medium; and reacting the organic composition with theshort chain alcohol or water in the presence of a catalyst at atemperature from about 20° C. to about 200° C. and a pressure from about150 psig to about 4000 psig, wherein the reaction occurs in a singlephase to produce a final product comprising an alkyl ester and glyceroland wherein the glycerol leaves the single phase as the glycerol isformed; wherein the critical fluid medium is selected such that areaction temperature is within about 20% of a critical temperature ofthe critical fluid medium and a reaction pressure is within about 0.5 toabout 15 times a critical pressure of the critical fluid medium asmodified by a co-solvent.
 13. A process for producing alkyl estersuseful in biofuels and lubricants, the process comprising: providing anorganic composition comprising at least one component selected from thegroup consisting: dissolving the organic composition and a C₁-C₄ shortchain alcohol or water into a critical fluid medium, wherein thecritical fluid medium is at least one fluid selected from the groupconsisting of carbon dioxide, sulfur dioxide, methane, ethane, andpropane, and mixtures thereof, the critical fluid medium solubilizingthe organic composition and the C₁-C₄ short chain alcohol or water intoa single phase; reacting the organic composition with the C₁-C₄ shortchain alcohol or water in the presence of a catalyst in the single phaseto produce a final product stream comprising an alkyl ester andglycerol; separating the glycerol from the final product stream bymodifying a temperature and pressure of the critical fluid medium; andseparating the alkyl ester from the critical fluid medium by modifyingthe temperature and pressure of the critical fluid medium.